Subwoofers: A Brief Look at the Effectiveness of Using a Subwoofer in a Music System

Before turning to the review of the NHT subwoofer (review to be published next week) let me give you an overview of the merits of a subwoofer in a context restricted solely to music or restricted to music as part of a soundtrack.

We are faced with two questions: does the subwoofer go low enough; and, how loud must it play with inaudible distortion until it crosses-over to the main speakers.

The URL below provides an interactive presentation of the frequency range for standard acoustical instruments:

The lowest note for A concert piano is 27Hz, (nineteenth-century pipe organs went lower). An organ of Bach’s time would not extend to 30Hz. Note that while it may sound very low, standard percussion instruments do not go very deep, although they are capable of producing significant sound pressure.

A floor-standing speaker (9″ to 10.5″ equivalent cone area) will have a -3dB anechoic response ranging from 40 – 60 Hz (add 5 -15 Hz or so for a bookshelf speaker with 6.5″ woofers). The 10% distortion point in an anechoic chamber for 100dB SPL RMS (1 meter) will also occur in this frequency range.

Note that you cannot directly compare the equivalent cone area of the speakers in the passive floor stander to an active subwoofer with the same sized driver. The subwoofer is designed to provide long voice coil excursion to achieve high SPL at low frequencies. The driver is also designed to deal with heat dispersion. By virtue of its design, the driver often does not work well above 100Hz or so. The active electronics in the subwoofer allow for special protection modes to prevent the driver from becoming damaged. Overdriving a passive subwoofer below its passband can damage the driver.

The Subwoofer in the Listening Room

In a room, the -3dB point will be extended via model pressure variations of the room and effects of speaker boundary interface where the direct sound of the speaker interacts with sound reflected from the walls. This is usually called room gain, although Tom Nousaine, the veteran writer on the subject, points out in private e-mails there is no “passive gain” and prefers to call this “preservation of sound pressure” (private conversation). Tom’s site (www.nousaine.com) has numerous PDF downloads on subwoofers, among other interesting topics. As anyone familiar with in-room frequency response knows, it is anything but flat. A worthy set of graphs to illustrate the effect is available at:

The graphs isolate the room effects from the anechoic response of the speaker for different speaker and room placements. For the large room used in this test, there is an apparent preservation of sound pressure in the 35 – 50Hz range; this will not get the typical floor-stander flat to 27Hz. Smaller rooms offer more extension at lower frequencies for reasons best explained by Tom Nousaine:

The last article shows why the really small space of a car can produce so much SPL at the low end.

Room volume is not a criterion for picking a subwoofer, although some reviews have implied this. Our ears respond to sound pressure (the change in pressure referenced to the atmospheric pressure as a result of the sound wave propagating in space), not sound power. The acoustic intensity is the sound power per unit area. Acoustic intensity is found by squaring the rms pressure and dividing by the specific acoustic impedance. It can be seen that sound pressure in this acoustics equation acts like voltage in an electrical system. Sound power declines with area, but sound pressure declines with distance from the ear. The decline is 6dB with a doubling of distance between the speaker and the ear. In a small room, the speaker may be limited to 2 meters back, while a large room might allow a larger distance of 4 meters, especially for the case of movie room with multiple rows of sets. There is a 6dB reduction of sound power at that rear listening seat relative to the seat placement in the small room in this case. Pressure preservation does vary with room size as we saw in the data in the referenced websites.

The conclusion is the in-room response of a typical passive speaker will not be nearly flat at the desired 27Hz and it will not be possible to achieve 100dB SPL at 10% distortion at 1 meter at 27Hz. A subwoofer can provide added extension and dynamics for music scored with significant activity below 50Hz. The large-scale symphonies of Shostakovich are one example.

The complete set fills 11 CDs. The Haitink performance above is a well-recorded version of the symphonies at a bargain price. Just to cover the basic 20th century repertoire that calls for large orchestras easily requires a collection of more than 100 CDs.

Scores calling for 100 musicians will not have instruments playing in the frequency range a subwoofer is designed to reproduce across a whole movement, as evidenced if you unplug the main channels with a properly level-matched subwoofer active. Many adjust the subwoofer level control far higher than the appropriate level at which it should be set to hear all the money they spent on the subwoofer. If set correctly, one should never hear sound coming from the subwoofer with the crossover held below 80Hz (Toole Sound Reproduction section 13.3.8)

How Low Do We Need to Go?

The lowest frequency limits of standard and exotic extended range instruments (for example, contrabass flute) are shown at the following URL:

Some scores call for these instruments. The dominant instrument is the post 18th century organ with 32 foot pipes down to 16Hz. To hear the lowest sustained notes of the second movement of Saint-Saëns Organ symphony (No. 3) without significant attenuation, a subwoofer with more extension than what can typically be found in the sub $1000 price range is required The large SPLs produced by the giant organs also require a subwoofer with low distortion at these frequencies. The classic Munch recording is an excellent rendition with the organ well-reproduced in the second and final movements (RCA Living Stereo 61387). But, the Saint-Saëns is a rare example of a symphonic work with such a prominent organ part. Unless you listen to a significant amount of 19th and 20th century pieces for organ, the expense to get beyond 27Hz is hard to justify. As an example, only half this CD is the Organ Symphony. The CD also contains an exceptional rendition of Debussy’s La Mer where the lowest notes are 27Hz and higher.

I do not define the cannon shots found in the score of the 1812 Overture as an exotic instrument. To reproduce these, please refer to another article by Tom Nousaine:

In my opinion, everything that affects the sound of a subwoofer is fully characterized by its measurements. To paraphrase Peter Aczel a fast woofer is an oxymoron. If the subwoofer is not flat in the near-field you will hear it (see measurements section of the NHT B10d at SECRETS for more details). If they do not go as low as the material you want to play, you will hear it. If the woofer cannot achieve the SPL you want to reproduce at inaudible distortion levels, you will hear it.

Room effects are the most significant factor to impact the sound, not the subwoofer. This depends on the placement of the subwoofer and your seat (Toole Sound Reproduction Loudspeakers and Room Focal Press, 2008; chapters 12 and 13). For those with the Toole text, it is best to carefully review the sections on waterfall and other time-domain graphics. The text clearly indicates the parameters required to produce a plot with usable data is often not well understood (Section 13.5). Proper setup and interpretation of waterfall and similar plots are especially difficult to set up and interpret. Some measurement tools designed for consumer use make very nice looking plots but do not provide the user with the ability to set up the measurement correctly. When using such tools it is possible to convince oneself that large changes in the time domain can occur with no visible change in a properly setup frequency domain plot.

Frequency and time-domain response issues can be resolved at one’s seat optimally or several seats sub-optimally with precise room EQ, provided the subwoofer is relatively flat and the deployment in the room is done thoughtfully. Room EQs that provide frequency plots on a PC come in handy to ensure the subwoofer is properly placed before the electronic correction is applied.

In difficult rooms, multiple subwoofers may be needed. Passive room absorbers can also assist, but these are large and expensive if they are to have any effect below 100Hz. Multiple subwoofers are especially useful when a flat frequency response over multiple seats is desired (Chapter 13.3 of the Toole text).

The Under-reported Issue When Adding a Subwoofer

By far, the most significant sonic issue for acoustic music when a subwoofer is deployed occurs around the crossover from the subwoofer to the main speaker. Adding a subwoofer to a system using a standard bass management system without a room EQ in the loop (sub and main channels) will degrade the flatness of the response in this critical area. At a minimum, the in-room transition band of the low-pass and high-pass filters should track the shape of a fourth-order Linkwitz–Riley filter to about 20dB down. With a 4th LR at 80Hz crossover, both speakers will be active between 60Hz and 110Hz (-10 dB points), a frequency range that is populated with a variety of common instruments playing throughout the score.

The standard bass management system provides only a second-order filter for the main speaker which will not even sum to flat even in an anechoic chamber. When the standard bass management system was developed (mid 90s) it was assumed the main channel speaker would roll off -12dB exactly below the crossover frequency to create the 4th order filter. This is never going to happen. Room EQs have to provide additional correction for the main speaker channel to have the correct transition and stop band associated with a 4th order high pass filter. Different main channel speakers will require a custom filter synthesized by the adaptive room equalizer. Once the EQ performs this function, the system is flat in an anechoic chamber, but this is only a small part of what the room EQ must do.

Without additional electronic equalization, the room effects (uneven sound pressure preservation) corrupt the shape of the low-pass and high-pass filter transition bands. In the graph below, the black curve shows the low-pass filter electrical response of the standard bass management systems applied to the full range input. The red curve is the room EQ filter’s response in cascade with the LPF.

The in-room high-pass filter transition band of the main speaker is as critical as the in-room low-pass rolloff of the subwoofer. Only an advanced electronic room EQ in a Pre/Pro or AVR can provide the required fourth-order Linkwitz – Riley response at the listening seat for the main channel. Some subs have a room EQ function, but this is only half the problem, even if the EQ works well. Likewise, multiple subwoofer techniques may expend the optimal area for the low pass filters transition and stop band but we can have only one main speaker per channel. Note that even the best room EQ can generate the optimum main channel response in the crossover area for only one listening seat. Passive room treatment can help but it is expensive since it must be effective to 60Hz (-10dB point for the main channel with an 80Hz crossover).

The crossover frequency must also be under the user’s control. The user must select a crossover point high enough that the subwoofer enters before the main channel speaker’s distortion starts to increase. At the same time, the crossover should be as low as possible to prevent localization of the subwoofer. Some electronic room equalizers will not allow the crossover frequency to be adjusted. This reduces the size of the filter needed to create a smooth response in the crossover, which can allow for a less expensive DSP to be used. Since no room EQ system makes distortion measurements, the automatic selection is often wrong.

Smaller speakers, as would be found in a smaller room, require a higher crossover point which makes frequency response deviations in the crossover even more audible. Unfortunately, smaller rooms often reduce the ability to move the main speaker, subwoofer and listening seat to reduce room effect in 75Hz -135Hz (100Hz crossover) region before EQ. Making an inaudible join between subwoofer and satellite is thus more difficult than a larger room.

Higher order crossover networks can reduce the overlap, which is especially useful with small main channel speakers (5 inch woofer). The discontinued NHT Xd used an 8th order filter. For a 100Hz crossover, the interaction between the main channel and subwoofer would be between 86Hz and 115Hz, but even this small range is audible if an electronic room EQ is not deployed.

The stringent requirements outlined above apply to music reproduction, and may not be applicable to sound effects. Some lifestyle systems equipped with very small satellite speakers (roll-offs starting at 150Hz) and a subwoofer with a bandpass-like response peaking around 60Hz basically are designed to reproduce sound effects. With such a system, musical instruments almost disappear between 75 to 130Hz.

Conclusions

In summary, a subwoofer enables realistic reproduction of the fundamentals of certain musical instruments. The cost is system complexity, including the addition of an excellent room EQ to achieve frequency response flatness in the 60Hz – 135Hz range. If missing this mark for the fundamentals of a few notes at the lowest end of the scale is not of concern, then you may decide to forgo the complexity and cost of the subwoofer and room EQ for music reproduction. To achieve the full range and power of all acoustic instruments necessitates a subwoofer. The NHT B10d at SECRETS is a welcome adjunct to reaching this goal.

David A Rich received his MSEE from Columbia University and Ph.D. in Electrical Engineering from Polytechnic University of NYU. He specializes in analog and mixed-signal integrated circuit design, with a portfolio spanning from audio ICs for Air Force One to RF ICs for wireless cell phones. Holder of fourteen patents, David's industrial experience includes ten years at Bell Laboratories where he rose to Technical Manager. He has taught graduate and undergraduate courses in integrated electronics and electro-acoustics. David's interest in audio began very early under the influence of his father, who was a recording engineer during the halcyon days of RCA Records. He gained an early understanding of electronics building Echo, Heath, and Dynakits. At the start of quadraphonic sound, David was building decoders based on designs in Popular Electronics. During his teen years, he spent more time in the back of TVs than watching movies at the front of the set. He still has limited interest in things video.
David has been a professional reviewer of audio equipment for more than two decades, serving as Technical Editor for The Audio Critic and Sensible Sound. He is a member of the Audio Engineering Society High-resolution Audio Technical Committee.
Collecting classical recordings has been a passion that has run concurrently with involvement in audio hardware. David has a special interest in early- and mid-20th century American tonal composers. He is an avid fan of chamber music and leads the Selection Committee for the Bethlehem (Pennsylvania) Chamber Music Society Concert Series.

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